1. Field of the Invention
The present invention relates to an optical disk apparatus and an optical aberration correcting method and is suitably applied to, for example, an optical disk apparatus that records a hologram on an optical disk.
2. Description of the Related Art
An optical disk apparatus that irradiates a light beam on an optical disk such as a CD (Compact Disc), a DVD (Digital Versatile Disc), or a Blu-ray Disc®, hereinafter referred to as BD) and reads reflected light of the light beam to reproduce information has been widely spread.
Such an optical disk apparatus in the past irradiates a light beam on the optical disk and changes a local reflectance and the like of the optical disk to thereby record information.
Concerning the optical disk, it is known that a size of a light spot formed on the optical disk is roughly given by λ/NA (λ: wavelength of a light beams, NA: numerical aperture) and resolution is proportional to this value. For example, details of the BD that can record data of about 25 [GB] on an optical disk having a diameter of 120 [mm] are described in Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys., 39, 756 (2000).
Various kinds of information including various contents such as music contents and video contents and various data for computers are recorded on the optical disk. In particular, in recent years, an amount of information has increased because of high definition of videos, high sound quality of music, and the like. An increase in the number of contents recorded on one optical disk is demanded. Thus, a further increase in capacity of the optical disk is demanded.
Thus, there is also proposed a method of superimposing recording layers in one optical disk to thereby increase a recording capacity in the disk (see, for example, I. Ichimura et al, Technical Digest of ISOM' 04, pp 52, Oct. 11 to 15, 2005, Jeju Korea).
On the other hand, as a method of recording information on an optical disk, there is also proposed an optical disk apparatus employing a hologram (see, for example, R. R. McLeod et al., “Microholographic multilayer optical disk data storage,” Appl. Opt., Vol. 44, 2005, pp 3197).
For example, as shown in FIG. 1, an optical disk apparatus 1 once condenses a light beam in an optical disk 8 made of photopolymer or the like, a refractive index of which changes according to the intensity of light irradiated thereon, from an optical head 7 and thereafter condenses a light beam in an identical focal point position again from the opposite direction using a reflecting device 9 provided on a rear surface side (a lower side in FIG. 1) of the optical disk 8.
The optical disk apparatus 1 causes a laser 2 to emit a light beam formed by a laser beam, modulates a light wave of the light beam with an acoustic optical modulator 3, and converts the light beam into parallel light with a collimator lens 4. Subsequently, the light beam is transmitted through a polarization beam splitter 5, converted from linear polarized light into circular polarized light by a quarter-wave plate 6, and then made incident on the optical head 7.
The optical head 7 can perform recording and reproduction of information. The optical head 7 reflects the light beam with a mirror 7A, condenses the light beam with an object lens 7B, and irradiates the light beam on the optical disk 8 rotated by a spindle motor (not shown).
At this point, the light beam is once focused in the inside of the optical disk 8, then reflected by the reflecting device 9 arranged on the rear surface side of the optical disk 8, and condensed at an identical focal point in the inside of the optical disk 8 from the rear surface side of the optical disk 8. The reflecting device 9 includes a condenser lens 9A, a shutter 9B, a condenser lens 9C, and a reflection mirror 9D.
As a result, as shown in FIG. 2A, a standing wave is generated in a focal point position of the light beam. A recording mark RM that generally assumes a shape obtained by sticking two cones together on bottom surfaces thereof and includes a hologram having a small light spot size is formed. In this way, this recording mark RM is recorded as information.
In recording plural recording marks RM in the inside of the optical disk 8, the optical disk apparatus 1 rotates the optical disk 8 and arranges the respective recording marks RM along a track of a concentric circle shape or a spiral shape to thereby form one mark recording layer. The optical disk apparatus 1 can record the respective recording marks RM in such a manner as to superimposing plural mark recording layers by adjusting the focal point position of the light beam.
Consequently, the optical disk 8 has a multilayer structure having the plural mark recording layers in the inside thereof. For example, as shown in FIG. 2B, in the optical disk 8, a distance between the recording marks RM (a mark pitch) p1 is 1.5 [μm], a distance between tracks (a track pitch) p2 is 2 [μm], and a distance between layers p3 is 22.5 [μm].
In reproducing information from the optical disk 8 in which the recording marks RM are recorded, the optical disk apparatus 1 closes a shutter 9B of the reflecting device 9 to prevent a light beam from being irradiated on the optical disk 8 from the rear surface side thereof.
In this case, the optical disk apparatus 1 irradiates, with the optical head 7, the light beam on the recording marks RM in the optical disk 8 and makes a reproduced light beam generated from the recording marks RM incident on the optical head 7. This reproduced light beam is converted from circular polarized light into linear polarized light by the quarter-wave plate 6 and reflected by the polarization beam splitter 5. The reproduced light beam is condensed by the condenser lens 10 and irradiated on a photodetector 12 via a pinhole 11.
The optical disk apparatus 1 detects an amount of light of the reproduced light beam with the photodetector 12 and reproduces the information on the basis of a result of the detection.